The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to a gradient coil set having higher order gradient coils arranged about a gradient coil and constructed to allow improved field-of-view adjustment.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The use of gradient coils to generate a gradient field about the bore of a magnet for imaging is known in the art of nuclear magnetic resonance imaging. Generally, a patient is positioned on an examination table and inserted into a bore of a magnet. The magnet provides a uniform magnetic field B0 throughout the bore. The gradient coils extend around the bore and are energized to impose time varying magnetic fields on the uniform magnetic field.
Conventional gradient coils have a fixed field-of-view (FOV). It is generally well known that the larger the FOV, the lower the efficiency rating for that respective coil. That is, a gradient coil with a large FOV requires more power to produce a given gradient strength than a gradient coil with a small FOV. Since coil inductance increases with FOV size, the slew rate of a gradient coil with a large FOV is reduced for a given power supply. Additionally, since high dB/dt is associated with larger FOVs, which can result in peripheral nerve stimulation, imaging protocols requiring high gradient power and high slew rate are generally performed on MRI systems equipped with a small FOV gradient set.
A number of improvements have been developed to provide more than one FOV for the gradient field in MRI systems. One approach is to integrate two sets of gradient coils on one system to provide, at most, three distinct FOV sizes. Manufacturing a coil with this approach is relatively straight forward, however, coil efficiency is greatly reduced. Another approach requires the disabling or enabling of certain parts of the gradient coil windings to adjust the FOV. With this approach, coil efficiency is improved, but such systems require switches within the coil windings to enable or disable part of the windings thereby increasing manufacturing and implementation complexity.
It would therefore be desirable to have a system and method of MR imaging incorporating a gradient coil set capable of infinitely variable FOV adjustments that maintain coil efficiency without the need for costly switching.
The present invention provides a system and method of MR imaging implementing a gradient coil set with variable FOV adjustments overcoming the aforementioned drawbacks.
In accordance with one aspect of the invention, a coil assembly with flexible FOV for use with an MR imaging system comprises a gradient coil disposed about an imaging axis to produce a gradient field. The gradient coil has a first end and a second end. The coil assembly further includes a higher order gradient coil, a first portion of the higher order gradient coil positioned overlapping at least a portion of the first end of the gradient coil and a second portion of the higher order gradient coil positioned overlapping at least a portion of the second end of the gradient coil.
In accordance with another aspect of the invention, an MRI apparatus to vary the imaging FOV is disclosed. The apparatus includes an MRI system having a number of gradient coils positioned about a bore of a magnet to produce a polarizing magnetic field and an RF transceiver system and an RF modulator controlled by a pulse control module to transmit RF signals to an RF coil assembly to acquire MR images. The MRI apparatus also includes at least one higher order gradient coil disposed about an imaging axis and configured to adjust gradient field linearity and generally disposed about the plurality of gradient coils and a control to vary the FOV by modulating current in the at least one higher order gradient coil.
In accordance with a further aspect of the invention, a method of manufacturing a coil assembly for an MR imaging apparatus is provided. The method includes positioning a primary gradient coil about a bore of a magnet wherein the primary gradient coil has a first and a second end. The method further includes the step of positioning a first portion of a higher order gradient coil circumferentially about at least a portion of the first end and positioning a second portion of the higher order gradient coil circumferentially about at least a portion of the second end. The first and the second portions are positioned such that the first portion overlaps the first end of the primary gradient coil and the second portion overlaps the second end of the primary gradient coil.
In accordance with yet another aspect of the present invention, a method of producing a variable FOV with an MR system having a gradient coil to produce a gradient field and an independent higher order gradient coil is provided. The method includes the step of energizing the gradient coils and the higher order gradient coil with a current to produce a gradient field of magnetic field gradients to spatially encode acquired signals in an FOV. The method also includes modulating the current in the independent higher order gradient coil to vary the FOV.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.